A range finder enables the measurement of the range separating it from a target. An optical range finder uses the propagation of light as the means of measurement. The range finder is composed of a transmitter and a receiver. It emits light in the direction of the target and detects a fraction of this light returned by the target. The range is obtained on the basis of the round-trip propagation time of the light from the transmitter to the receiver. The transmission is temporally modulated. The transmitted light transports this modulation to the target. The target reflects or backscatters this light. A fraction of this returned light transports the modulation to the receiver of the range finder. This temporal modulation enables the identification of the departure of the pulse and the identification of its return by the receiver. The time elapsed between these two events enables the range between the range finder and the target to be calculated on the basis of the propagation speed of the light in the environments through which it passes.
According to the various situations involved, the design of the range finder seeks to achieve the best compromise. Numerous technical parameters are to be optimized: the transmission wavelength(s) and their spectral widths, the geometry of the transmission (diameter and divergence), the geometry of the reception (pupil diameter and receive field), and the temporal modulation profile. The compromises also take into account the means for aiming the transmission and the reception in the direction of the target, and the disturbances produced by the thermal and mechanical environments. In some cases, the transmission and reception share the same optical pupil.
The typical design of a range finder comprises:                a transmitter device including a transmitter and its beam-shaping optical system,        a receiver device, as shown in FIG. 1, which includes an optical system 10 for collecting and focusing the flow coming from the target and a detector 4. The field of the receiver is limited in the focusing plane by the dimension of the sensitive zone of the detector, referred to as the detection zone, or by an optical system for transporting the light to the detector (via fiber 2, for example). A spectral filtering is incorporated in the path.        
The fraction of transmitted light collected by the optical system of the receive device is very weak. The ways of increasing performance are:                either being capable of detecting weaker signals,        or increasing the illuminance of the target.        
The solutions currently proposed for detecting a weak flow are derived from one of the following categories, based on:                an increase in the repetition rate of the transmitted pulses which enables the use of post-integration, but whose main constraints are an increase in the size of the laser, in its power consumption and in the measurement duration;        an increase in the surface of the receive pupil, the latter being limited by a size constraint;        a reduction in the noise sources. Detection of weaker signals is possible with the same detection technology. The noise sources are external or internal to the range finder. External noise is the noise whose source is the light (ambient solar illuminance, for example) reaching the detector but which was not transmitted by the transmitter. This noise source may be reduced by reducing the spectral width of the filter to the detriment of its transmission and its performance as a function of temperature. Internal sources are mainly linked to the dimensions of each detector and of the bandwidth required for the detection of the temporal modulation. The dimension of the detection zone determines the receive field. This dimension can be reduced, resulting in a reduction in the receive field with the risk of no longer being able to see the target;        an increase in the illuminance of the target. This increase may be obtained by increasing the power or the energy supplied by the transmitter, but this energy is limited by eye safety constraints, and this energy increase requires a larger transmit device;        a reduction in the divergence of the transmission increases the fraction of the flow intercepted by a target smaller than the illuminance spot level with it. This also occurs when only a part of the target makes a significant contribution. Conversely, however, there is the need to point the transmission of the laser with precision at the target, particularly in the case of small, isolated (for example aerial) target range measurement. The line of sight of the range finder is usually oriented with the aid of a camera enabling the target to be maintained on the line of sight during the tracking of the target. In the case of a low-divergence beam which only covers part of the target, a different problem may arise. The line of sight of the range finder is rarely in the direction of the most contributive part of the target in terms of the range measurement. This zone returns the greatest fraction of the transmission through reflection or backscatter to the reception. This part is not readily identifiable in an image which does not use the illuminance of the target by the range finder transmission. Therefore, in order to avoid a dramatic loss of performance when the line of sight is not exactly at the best location, the lesser evil is to distribute the transmission sufficiently over the target, i.e. to increase the divergence of the beam, to the detriment of the range.        
Ultimately, the detection performance is generally obtained to the detriment of the detection range.
Consequently, there still remains a need for a system which simultaneously meets all of the aforementioned requirements, i.e. detection of a weak flow and a long detection range.